The present invention relates to a magnet knife assembly for a toner developing device, comprising a support body and a ferromagnetic strip that is held between like poles of two permanent magnets and extends between these poles from a knife edge that faces outwardly of the support body to an inner edge facing inwardly of the support body, the strip being held such that an outer knife edge portion of the strip projects outwardly beyond the two magnets and is subject to a magnetic force that tends to urge the strip in a direction in which the knife edge portion projects further out of the magnets.
A magnet knife assembly of this type is used in toner developing devices for printers, copiers and the like for creating, along the knife edge, a localized strong and strongly divergent magnetic field, so that, when magnetically attractable toner particles are supplied into that field, they will form a magnetic brush extending along the knife edge and across an image forming medium so as to assist in the transfer of the toner onto the image forming medium.
Typically, the magnet knife assembly is held stationary relative to the path along which the image forming medium is moved, and is surrounded by a thin sleeve, so that the knife edge faces the internal surface of the sleeve and the magnetic field penetrates through the wall of the sleeve towards the image forming medium. Toner particles may then be supplied to the magnetic field by distributing the toner on the surface of the sleeve and rotating the sleeve so that the toner approaches the magnet field created by the knife edge.
In order to obtain a high and constant quality of the developed image, certain parameters of the magnet field created at the knife edge must fulfil a number of criteria. For example, the absolute strength of the magnetic field directly above the knife edge should be relatively high, and the field should further be highly inhomogeneous, i.e., the gradient of the radial component of the magnetic field above the knife edge should also be high. Moreover, the angle a which the magnetic field vector forms with the surface of the sleeve (the tangent plane thereof at the position above the knife edge) should be relatively high and should be larger than 45° over a certain distance in the circumferential direction of the sleeve.
Magnet knife assemblies of the type indicated above are disclosed in EP 0310209A, EP 0298532A and EP 0773484A.
EP 0304983A discloses another magnet knife assembly of this type that was optimised in view of the above requirements. In this magnet knife assembly, the two permanent magnets have rectangular cross-sections that may be chamfered on the sides facing away from the ferromagnetic strip interposed therebetween. The plane of the strip is inclined at an angle of about 15° relative to the radial direction of the sleeve. It has been found that, for this configuration, the absolute strength and the inhomogenity of the magnetic field above the knife edge increases when the length of the strip (essentially in the radial direction of the sleeve) is reduced. For that reason, the length of the strip is shorter than the length of the two magnets. This has the consequence that the magnetic force tends to push the knife edge portion of the strip away from the magnets, i.e., tends to cause the strip to project further from the magnets.
For this reason, it is necessary in the known assembly that the strip is mechanically fixed at a support structure that carries the two magnets, e.g., by gluing the strip and the magnets to the support structure with an adhesive, by clamping the strip and/or the magnets with fastening screws, and the like. However, the necessity to fix the strip and the magnets in their desired positions requires cumbersome procedures and therefore increases the production costs for the magnet knife assembly as a whole. Moreover, differential thermal expansion of the magnet knife assembly and the support structure may lead to undesired mechanical strains and distortions.